1. Field of The Invention
The present invention pertains to gas-liquid contacting towers and, more particularly, to an improved downcomer-tray assembly incorporating an active bridge between adjacent downcomers, an active support ring within the tower, and active washers secured thereto for increasing the effective active area of the tray.
2. History of the Prior Art
Distillation columns are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact columns utilize either trays, packing or combinations of each. In recent years the trend has been to replace the so-called "bubble caps" by sieve and valve trays in most tray column designs. Additionally, random (dumped) or structured packings have been utilized in combination with the trays in order to effect improved separation of the components in the stream.
Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up. Another type of vapor and liquid contact apparatus, namely structured high efficiency packing, has also become popular for certain applications. Such packing is energy efficient because it has low pressure drop and low liquid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
Fractionation column trays generally come in one of two configurations: cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the tower. In cross-flow trays, vapor ascends through the apertures and contacts the liquid moving across the tray; through the "active" area thereof. In the active area, liquid and vapor mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer. The liquid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. The location of the downcomers determine the flow pattern of the liquid. If there are two Inlet Downcomers and the liquid is split into two streams over each tray, it is called a two pass tray. If there is only one Inlet and one Outlet Downcomer on opposite sides of the tray, it is called a single pass tray. For two or more passes, the tray is often referred to as a Multipass Tray. The number of passes generally increases as the required (design) liquid rate increases. It is the active area of the tray, however, which is of critical concern.
Not all areas of a tray are active for vapor-liquid contact. For example, the areas along the perimeter of the tray and under the Inlet Downcomer are generally solid regions. To attempt to gain more area of the tray for vapor/liquid contact, downcomers are often sloped. The maximum vapor/liquid handling capacity of the tray generally increases with an increase in the active or Bubbling Area. There is, however, a limit as to how far one can slope the downcomer(s) in order to increase the Bubbling Area; otherwise the channel will become too small. This can restrict the flow of the liquid and/or restrict the disengagement of vapor retained in the liquid, cause liquid to back up in the downcomer, and thus prematurely limit the normal maximum vapor/liquid handling capacity of the tray.
A variation for increasing the Bubbling Area and hence vapor/liquid handling capacity is a multiple downcomer tray. There are usually many box shaped vertical channels installed in a symmetrical pattern across the tray to direct liquid onto and off of the tray. The downcomers do not extend all the way to the tray below but stop short of the tray by a predetermined distance which is limited by a sufficient space to permit disengagement of any vapor retained in the liquid entering the Exit Downcomer. The downcomer pattern may be rotated 90 or 180 degrees between successive trays. The bottom of the boxes is solid except for slots that direct the liquid onto the tray below. Such trays fall into the category of Multipass Trays which are usually used for high liquid rates. A critical feature in such trays is the available active area of the tray. Designs for increasing this active area are thus of major import in tray fabrication.
Various techniques have been developed for increasing the tray active area in process column designs. For example, U.S. Pat. No. 4,956,127, assigned to the assignee of the present invention, illustrates a tray design with a raised active area disposed beneath the downcomer inlet for increasing the active area of the tray. U.S. Pat. No. 5,164,125, also assigned to the assignee of the present invention, again addresses a downcomer-tray assembly for vapor liquid contact towers featuring improved downcomer and tray designs for enhancing the active area of the tray as well as the balance of liquid flow thereon. The balance of liquid flow is of primary significance in such trays. As set forth in U.S. Pat. No. 5,192,466, also assigned to the assignee of the present invention, methods of and apparatus for flow promotion and effective balance of flow upon a tray is an important design feature. When flow is uneven or stagnated, the efficiency of the chemical process column is drastically reduced. For this reason, these and other innovations in the downcomer-tray area have received considerable attention.
In addition to the above, the technology of gas-liquid contact addresses many other performance issues. Examples are seen in several prior art patents, which include U.S. Pat. No. 3,959,419, 4,604,247 and 4,597,916, each assigned to the assignee of the present invention and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. A particularly relevant reference is seen in U.S. Pat. No. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating vapor ascension therethrough.
U.S. Pat. No. 4,550,000 assigned to Shell Oil Company teaches apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings mounted to the top of the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances improve tray efficiency within the confines of prior art structures. Likewise, U.S. Pat. No. 4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle tray tower. The operational parameters of high gas-liquid contact efficiency and the need for low pressure loss are set forth. Such references are useful in illustrating the need for high efficiency vapor liquid contact in tray process towers. U.S. Pat. No. 4,504,426 issued to Carl T. Chuang et. al. and assigned to Atomic Energy of Canada Limited is yet another example of gas-liquid contacting apparatus. This reference likewise teaches the multitude of advantages in improving efficiency in fractionation and modifications in downcomer-tray designs. The perforated area of the tray is extended beneath the downcomer with between 0 to 25% less perforation area.
Yet another reference is seen in U.S. Pat. No. 3,410,540 issued to W. Bruckert in 1968. A downcomer outlet baffle is therein shown to control the discharge of liquid therefrom. The baffle may include either a static seal or dynamic seal. In this regard the openings from the downcomer are sufficiently small to control discharge and may be larger than the tray perforations and of circular or rectangular shape. The transient forces which may disrupt the operation of a downcomer are also more fully elaborated therein. These forces and related vapor-liquid flow problems must be considered for each application in which a downcomer feeds an underlying tray.
Yet a further reference addressing downcomer tray assemblies and methods of mixing vapor with liquid from a discharge area of a downcomer is set forth and shown in U.S. Pat. No. 4,956,127 (the '127 Patent) assigned to the assignee of the present invention. In the '127 Patent, a raised active inlet area as set forth and shown, which inlet area is provided for the venting of vapor from the tray therebeneath. The raised inlet area reduces fluid pressure of the vapor to facilitate the flow of ascending vapor therethrough. A series of louvers disposed on the raised active inlet area selectively directs the upward flow of vapor into the liquid region below the downcomer to generate more efficient vapor liquid contact and reduced back mixing across the tray. The discharge of liquid from the downcomer onto the raised active inlet area, though effective, has been shown to result in weeping as the discharged liquid from the downcomer passes through the apertures of the active inlet area. Additionally, the liquid splashing outwardly from the downcomer increases the frothiness thereof and causes liquid drops to be more easily suspended.
As set forth above, the effectiveness of downcomer-tray operation is directly related to fluid flow configurations. When downcomer weirs and other structural aspects of the tray inhibit either vapor or liquid flow, tower efficiency is reduced. In multiple downcomer tray assemblies, the tray decks are literally divided by the downcomer. This tray division can result in unequal flow on opposite sides of the downcomer. Likewise, structural members, such as support beams disposed under tray areas, can interfere with ascending vapor flow. Other structural members such as tray hold down and securement devices often disposed on tray perimeters may likewise present solid, non-active tray areas which prevent vapor flow. Such inactive, solid areas reduce tray efficiency as described above. It would be an advantage therefore to provide a tray assembly addressing the problems of both liquid and vapor flow uniformity in a configuration which maximizes the active area of the tray and simplifies certain structural aspects therein to maximize the efficiency.
Such a downcomer-tray assembly is provided by the present invention wherein a perforated column support ring and perforated washers are secured along the tray perimeter to increase the active area of the tray. Additionally, a structural baffle system may be used to support the downcomers, positioning them above a tray area incorporating venting chambers upstanding on the tray deck therebelow and active "bridges" therebetween. The absence of structural support beams beneath the tray deck in conjunction with the active bridges and tray perimeter improve the efficiency of the column. The baffle support may be further constructed in multiple downcomer configurations to permit liquid flow thereacross for flow equalization upon the tray.